Might sound daft, but no harm in asking lol.
Will flashing a network locked phone with a custom/generic rom automatically unlock the phone?

Basically i took my sony z3 (02) to be unlocked on monday. I paid £20 and the guy took a photo of the imei number and told me to ring back in a couple of days for the code.
Thursday afternoon i rung and he said that they cant get a code for it, but if i bring it in and leave it for a day, he will sort it at no extra cost.
Im just wondering how he plans to do it, as the only other option im aware of is flashing.
I just want to be armed before i go in.
TIA.

Yes from memory iirc when you ad a custom rom to it without it being unlocked then you can't unlock it via the current rom would have to revert to stock

Unlock codes are about £15 on the bay

I may be wrong but if your firmware allows a bootlloader unlock via adb fastboot then you can flash a custom ROM. My first couple of phones were Samsung devices from Orange and I flashed then with no unlock code needed with generic EU firnware. Can't recall the exact steps but it involved using adb. I now always buy unlocked SIM free so don't have to worry about carrier unlocking.